Electric halide vapor heater

ABSTRACT

A process and apparatus for heating the vapor of a halide of an element selected from the group consisting of titanium, silicon, aluminum, zirconium and iron and mixtures of such halides. The vapor to be heated is passed through a plurality of ducts which are arranged in parallel relation with one another and in which the vapor comes into contact with a plurality of resistance elements provided in the ducts. The resistance elements are formed of platinum or an alloy of platinum with rhodium, ruthenium or iridium. The inner surface of the ducts are formed of a nonmetallic refractory material which is inert with respect to the halide vapor to be heated. The vapor, in passing through the ducts, is heated by convection from the resistance elements and by radiation from the walls of the ducts.

United States Patent Inventor Anthony Horton Welwyn Garden City, England Appl. N0. 810,190 Filed Mar. 25, 1969 Patented Dec. 7, 1971 Assignee Laporte Titanium Limited London, England Priority Apr. 3, 1968 Great Britain 15,983/68 ELECTRIC HALIDE VAPOR HEATER 2 Claims, 5 Drawing Figs.

U.S. Cl 219/374, 23/288 J, 219/307, 219/375, 2l9/382, 219/550 Int. Cl. 05b 3/00, F24h 3/04 Field of Search 2 l 9/550, 368, 373-376, 381, 382, 299, 298, 319, 306, 307, 308, 309; 23/288 1, 288 K References Cited UNITED STATES PATENTS 6/l9l6 Pearson 2l9 /309 X Primary Examiner-A. Bartis Attorneywilliam G. Addison ABSTRACT: A process and apparatus for heating the vapor of a halide of an element selected from the group consisting of titanium, silicon, aluminum, zirconium and iron and mixtures of such halides. The vapor to be heated is passed through a plurality of ducts which are arranged in parallel relation with one another and in which the vapor comes into contact with a plurality of resistance elements provided in the ducts. The resistance elements are formed of platinum or an alloy of platinum with rhodium, ruthenium or iridium. The inner surface of the ducts are formed of a nonmetallic refractory material which is inert with respect to the halide vapor to be heated. The vapor, in passing through the ducts, is heated by convection from the resistance elements and by radiation from the walls of the ducts.

1 ELECTRIC IIALIDE VAPOR IIEATER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process and apparatus for heating a vapor of the halides (excluding the fluoride) of titanium, silicon, aluminum, zirconium and iron.

2. Description of the Prior Art Problems arise in the heating of halide vapors of titanium, silicon, aluminum, zirconium and iron up to temperatures of the order of 700 C. to l,000 C. as, for example, to enable them to undergo oxidation reactions with oxidizing gases, because of the corrosive nature of the halides at such temperatures.

In apparatus for heating halide vapors up to such temperatures, platinum or a suitable alloy of platinum may be used for portions of the apparatus which are in contact with the hot halide gas because its rate of corrosion at these temperatures is tolerable, but, because of the high cost of platinum and its alloys, the use of these materials for this purpose is usually only economically justifiable if the quantity of these materials used can be kept down to a relatively low level.

When a hot metal is used to heat a gas, the transfer of heat from the metal to the gas is usually effected by convection and it is of course desirable, if the quantity of metal is required is to be kept low, that the surface area of the metal that is contacted by the gas per unit mass of metal should be as high as possible. Considerations of mechanical strength, however, impose a limit on the extent to which the ratio of the exposed surface area to the mass of the metal can be increased.

SUMMARY OF THE INVENTION The present invention provides a process for heating the vapor of a halide (excluding the fluoride) of one of the elements titanium, silicon, aluminum, zirconium and iron, or a mixture of more than one of such halides, which comprises causing the vapor to come into contact with a plurality of resistance elements made of platinum or an alloy of platinum with rhodium, ruthenium or iridium, located in a plurality of ducts arranged in parallel with one another, the inner surface of each duct being of a nonmetallic refractory material inert with respect to the halide vapor, and passing an electric current through the resistance elements.

This invention also provides apparatus for heating the vapor of a halide (excluding the fluoride) of one of the elements titanium, silicon, aluminum, zirconium and iron, or a mixture of more than one of such halides, which comprises a plurality of resistance elements made of platinum or an alloy of platinum with rhodium, ruthenium-or iridium, and a plurality of ducts arranged in parallel with one another and in which the resistance elements are located, the inner surface of each duct being of a nonmetallic refractory material.

Advantageously, the ducts in which the resistance elements are located are formed in a plurality of nonmetallic refractory supporting members so arranged in a conduit, the inner surface of which is of a nonmetallic refractory material, that the ducts of each of the supporting members are in register with the ducts of the or each adjacent supporting member.

In the process and apparatus of the invention, heat is transferred to the vapor not only by direct convection from the resistance elements but also by a process involving radiation of heat from the resistance elements. The heat radiated from each resistance element raises the temperature of the wall of the duct in which that element is located and convection of heat from the duct wall to the vapor results. The rate of heat transference to the vapor from a given mass of platinum is thereby increased and consequently the total quantity of platinum or platinum alloy needed can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS One form of apparatus suitable for heating a halide containing vapor such as a mixture of titanium tetrachloride vapor and aluminum chloride vapor prior to its vapor-phase oxidation in the manufacture of titanium dioxide incorporating alumina and constructed in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic vertical axial section through the apparatus;

FIG. 2 is a cross section through the apparatus taken on the line 2-2 in FIG. 1;

FIG. 3 is a diagrammatic vertical elevation on a larger scale than FIG. 2 of one of the plates forming a supporting member;

FIG. 4 is a plan view on a larger scale than FIG. 3 of part of the upper surface of the upper plate of one of the supporting members; and

FIG. 5 is a vertical section on a larger scale than FIG. 4 through a portion of either the upper or lower plate of each supporting member.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention there is provided a process and apparatus for heating the vapor of a halide, excluding the fluoride, of an element selected from the group consisting of titanium, silicon, aluminum, zirconium and iron and mixtures of such halides.

The present invention comprises a plurality of resistance heating elements, which are made of platinum or an alloy of platinum with rhodium, ruthenium or iridium, and a plurality of ducts arranged in parallel relation with one another in which the resistance heating elements are located. The vapor to be heated is passed through the ducts and therein comes into contact with the resistance heating elements to raise the temperature of the vapor.

Referring to FIG. 1 of the drawings, there is illustrated an embodiment of the apparatus of this invention which includes a cylindrical outer casing, which is mounted with its axis vertical and is indicated generally by the reference numeral 1. Except for its lower end portion 2, the outer casing 1 is provided with a lining 3 of a nonmetallic refractory material, the inner surface of the lining 3 defining a conduit 4 though which the mixture of titanium tetrachloride vapor and, if desired, other halide vapors to be heated can be passed upwardly.

At its lower end, the casing l is closed by a plate 5 which makes a gastight seal with an annular flange 6 formed at the lower end of the casing I. Supporting members 7 extend between the flange 6 and the lower end portion 2 of the sidewall of the casing I. A tubular inlet 8 permits the introduction of the vapor to be heated into the interior of the unlined portion 2 of the casing I and is provided, outside the casing, with a thermowell 9 to enable the temperature of the incoming vapor to be measured.

Near to the closed top of the casing 1, there is provided a tubular outlet 10, which is coaxial with a bore 11 formed in the lining 3. The inner end of the bore 11 opens into the upper end portion of the conduit 4 and a thermowell 12 is provided to enable the temperature of the heated vapor flowing out through the bore to be measured.

Situated within the conduit 4 is a column of circular ceramic plates which make a sliding fit within the conduit and are arranged to form four supporting members 13, 14. I5 and I6, one for each of four resistance elements 13, 14', 15 and I6. Each of the supporting members 13 to 16 is made up of lower and upper end plates 17 and a plurality of intermediate plates 18. Five additional plates 18 are provided, one at the bottom of the column, one between the supporting member of each pair of adjacent supporting members and one at the top of the column.

The column of plates 17 and 18 is supported by three flanges 19 which extend inwardly from the unlined portion 2 of the casing 1 immediately below the lining 3 and beyond the inner surface of the lining so that they abut against the under surface of the lowermost plate 18.

Each of the plates 17 and 18 is formed with a plurality of ducts 20 which extend through them in a vertical direction and the plates are so arranged that each duct 20 is in register with a duct 20 in the or each adjacent plate 17 or 18. Thus, the column of plates 17 and 18, is pierced by a plurality of ducts, through each of which the vapor to be heated can flow from one end of the conduit 4 to the other.

To ensure that the plates 17 and 18 are so orientated about their common axis that the ducts 20 of adjacent plates are in register with one another the upper surface of each plate 17 or 18 is formed with raised edge portions 21 and the lower surface is formed with corresponding depressed edge portions 22. When the plates 17 and 18 are placed on top of one another the raised portions 21 of each plate 17 or 18 fit into the depressed portions 22 of the plate 17 or 18 immediately above.

The ducts 20 in each plate 17 or 18 are arranged in three symmetrical arrays 23 so that on the upper or lower surface of each plate 17 or 18 each array 23 is in the shape of a rhombus as shown in FIG. 2. The ducts 20 are arranged in rows parallel to one side of the rhombus and in columns parallel to an adjacent side of the rhombus.

The end plates 17 differ from the plates 18 only in that the upper surface of each upper end plate 17 and the lower surface of each lower end plate 17 are formed with recesses 24 and 25 as shown in FIGS. 4 and 5. The recesses 24 are formed in alternate spaces between the ducts 20 in each row and the recesses 25 are formed in the spaces between the end ducts of adjacent rows of ducts.

In the embodiment illustrated in the drawings, the resistance elements 13', l4, l and 16 are in the form of platinum tape 26 (see FIG. 5) which pass up and down through the ducts of each of the supporting members 13 to 16. The recess 24 and 25 in the end plates 17 of each of the supporting members 13 to 16 accommodate the tape 26 where it passes from one duct to an adjacent duct so that it does not stand proud of the end surfaces of the supporting member (see FIG. 5 The tape 26 is of increased cross-sectional area where it passes laterally from one duct to an adjacent duct 20. This tends to avoid local overheating of the tape and thus permits higher current loadings with a consequent saving in platinum or platinum alloy. The ducts 20 are arranged in three separate arrays 23 to facilitate connection of the resistance elements for use with three-phase supply.

.The plates 18 situated between the supporting members act as spacers. Each of the supporting members 13 to 16 is also formed with a number of other ducts 27 formed between one of the arrays 23 and the edge portions 21 and 22 of the plates 17 and 18. Through these ducts 27 pass connecting wires (not shown) from one or more of the resistance elements to a terminal box 28 which is secured to the unlined portion 2 of the casing 1.

Each plate 17 and 18 is formed with a central bore and a ceramic rod 29 extends through these bores from one end of the column to the other, the column of plates being assembled on the ceramic rod in their correct relative positions before being inserted into the conduit 4.

The voltage applied to each resistance element is selected with a view to achieving such a stepwise decrease, in the direction of flow of the vapor, of the rate of generation of heat per unit surface area of the resistance elements as will minimize the temperature differences between the different elements when vapor is passed through the ducts.

The lengths of the supporting members 13 to 16 are determined by the requirement that they shall support the associated resistances elements with all the ducts occupied by the tape 26.

For all the tape 26 to be maintained as chose to the optimum temperature as possible it is of course desirable that the change in the temperature of the vapor over the length of each supporting member 13 to 16 shall be as small as possible, but the use of a large number of short elements leads to additional complexity and the use of four elements represents a compromise.

If desired, the apparatus may be modified in that each of the ceramic plates 18 may be replaced by plates similar in construction to the end plates 17.

The process and apparatus of the present invention is particularly well suited for use in the heating of vapors of titanium tetrachloride or a vaporous mixture of titanium tetrachloride and aluminum chloride which is to be used in the manufacture of titanium dioxide. It will be understood, however, that the process and apparatus of this invention may also be used for the heating of other halides of titanium, silicon, aluminum, zirconium and iron for use in other processes.

In the process and apparatus of the invention, heat is transferred to the vapor not only by direct convection from the resistance elements but also by a process involving radiation of heat from the resistance elements. The heat radiated from each resistance element raises the temperature of the wall of the duct in which that element is located and convection of heat from the duct wall to the vapor results. The rate of heat transference to the vapor from a given mass of platinum is thereby increased and consequently the total quantity of platinum or platinum alloy needed can be reduced.

As discussed hereinabove, preferably the resistance elements are so arranged and connected that the rate of generation of heat per unit surface area of the resistance element decreases in the direction of flow of vapor through the ducts.

The decrease (in a downstream direction) in the rate of generation of heat per unit surface area of the resistance elements tends to compensate for the decrease in a downstream direction of the cooling effect of the vapor on the resistance elements. Thus the temperature of the resistance elements can, without anywhere exceeding a given value, be at a higher average value than would otherwise be the case, and it is possible in this way to reduce further the total quantity of platinum or platinum alloy required.

The variation in the rate of generation of heat per unit surface area of the resistance elements may be achieved by applying different voltages to different resistance elements and/or by using resistance elements having different resistances. Where resistance elements having different resistances are used, they may be connected in parallel with one another and they may take the form of different lengths of wire or tape having the same resistance per unit length.

As discussed hereinabove, advantageously, the ducts in which the resistance elements are located are formed in a plurality of nonmetallic refractory supporting members so arranged in a conduit, the inner surface of which is of a nonmetallic refractory material, that the ducts of each of the supporting members are in register with the ducts of the or each adjacent supporting member.

Where different resistance elements are formed from different lengths of wire or tape, different supporting members may be of different lengths in the direction of flow of the vapor and/or the wires or tapes forming different resistance elements may pass through the supporting members different numbers of times.

Each supporting member may be composed of a plurality of plates secured together, as illustrated in FIG. 1, so that the plane of each plates is perpendicular to the direction of the path of flow of the vapor through the ducts, each plate being formed with a plurality of apertures which are so arranged that each aperture is in register with an aperture of each adjacent plate to form ducts which extend through the supporting member in the direction of flow of the vapor. Alternatively, each supporting member may, if desired, be composed of a plurality of plates secured together so that the plane of each plate is parallel to the direction of the path of flow of the vapor through the ducts, the adjacent surfaces of adjacent plates being each formed with a plurality of grooves which are so arranged as to form ducts which extend through the supporting member in the direction of flow of the vapor.

Advantageously, all the electrical connections to the resistance elements are made at the cold, upstream ends of the ducts.

While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification and this application is intended to cover any variations, uses or adaptations of the invention. It will therefore be recognized that the invention is not to be considered as limited to the precise embodiments shown and described but is to be interpreted as broadly as permitted by the appended claims.

What is claimed is:

1. An apparatus for heating the vapor of a halide of an element selected from the group consisting of titanium, silicon, aluminum, zirconium and iron and mixtures thereof which comprises an outer casing having an inlet and outlet,

a lining of refractory material located within said casing, the inner surface of said lining defining a conduit through which the vapor to be heated may be passed from said inlet to said outlet,

a plurality of supporting members, each member comprising a plurality of refractory plates positioned adjacent to one another in said conduit such that the plane of each plate is substantially perpendicular to the direction of the path of the vapor through said conduit, each of said plates having a plurality of apertures extending substantially parallel therethrough, at least the end plates in each of the supporting members having recessesin their end faces connecting said apertures in pairs, said plates being so positioned in said conduit that the apertures extending therethrough are in register with the apertures of each adjacent plate whereby a plurality of substantially continuous ducts are provided in said conduit;

each of said supporting members being individually provided with at least one corrosion-resistant resistance heating element extending through at least two of said apertures and being of increased cross section where it passes laterally through said recess in the end plates of said supporting members from one aperture to the adjacent aper ture of said pair, said resistance heating elements being so arranged and connected, that, in operation, the rate of generation of heat per unit surface area of the resistance heating elements decreases in the direction of flow of vapor through said conduit and means associated with each of said resistance heating elements adapted to supply electrical current thereto.

2. The apparatus defined in claim I in which said resistance heating elements are flexible and are made of a material selected from the group consisting of platinum and alloys of platinum with rhodium, ruthenium or iridium.

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1. An apparatus for heating the vapor of a halide of an element selected from the group consisting of titanium, silicon, aluminum, zirconium and iron and mixtures thereof which comprises an outer casing having an inlet and outlet, a lining of refractory material located within said casing, the inner surface of said lining defining a conduit through which the vapor to be heated may be passed from said inlet to said outlet, a plurality of supporting members, each member comprising a plurality of refractory plates positioned adjacent to one another in said conduit such that the plane of each plate is substantially perpendicular to the direction of the path of the vapor through said conduit, each of said plates having a plurality of apertures extending substantially parallel therethrough, at least the end plates in each of the supporting members having recesses in their end faces connecting said apertures in pairs, said plates being so positioned in said conduit that the apertures extending therethrough are in register with the apertures of each adjacent plate whereby a plurality of substantially continuous ducts are provided in said conduit; each of said supporting members being individually provided with at least one corrosion-resistant resistance heating element extending through at least two of said apertures and being of increased cross section where it passes laterally through said recess in the end plates of said supporting members from one aperture to the adjacent aperture of said pair, said resistance heating elements being so arranged and connected, that, in operation, the rate of generation of heat per unit surface area of the resistance heating elements decreases in the direction of flow of vapor through said conduit and means associated with each of said resistance heating elements adapted to supply electrical current thereto.
 2. The apparatus defined in claiM 1 in which said resistance heating elements are flexible and are made of a material selected from the group consisting of platinum and alloys of platinum with rhodium, ruthenium or iridium. 